1. Field of the invention
The present invention relates to flame retardant resin compositions, and more particularly, pertains to flame retardant thermoplastic polyester resin compositions which are excellent in mechanical properties, hygroscopicity and thermal stability.
2. Description of the Prior Art
Flame retardant thermoplastic polyester resin compositions using flame retarders with organic halides as their main components are problematical in thermal stability. Particularly, flame retardant polyethylene terephthalate resin compositions excellent in heat resistance require relatively high molding temperatures and undergo thermal degradation while being molded. For example, a flame retardant thermoplastic polyester resin compound improved in thermal stability was disclosed in Patent Laid Open No. Sho 57-195142. According to this patent, thermal stabilization is increased by forming an inert film for protection against hydrolysis of thermoplastic polyesters by treating with alkoxy silane the surface of antimony trioxide which is added to provide flame retardancy. However, as clarified by examples in said Gazette, the mechanical strengths, particularly the tensile strength, of the molded products illustrated are relatively low, such that they can hardly be said quite satisfactory in particular fields. The Patent Laid Open No. Sho 57-2357 states that the coated antimony oxide has no thermal stability which diminishes the commercial value of the product. In this Gazette, mention is made of particular compositions which are excellent in thermal stability. This Gazette says that particular substances which promote the rate of crystallization and antimonates of metals belonging to Groups I, II and VIII of the Periodic Table, when jointly used in flame retarder system, will enhance flame retardancy of the compositions, without badly affecting their thermal stability. Generally, crystallization promotors have a tendency to reduce mechanical strengths of resin compositions, and accordingly, their use is limited. It is widely known that oxides of antimony such as antimony trioxide, antimony pentoxide, etc. promote hydrolysis of thermoplastic polyesters. Use of antimonates is known to improve their hydrolyzing property. But antimonates have hygroscopicity such that when added to flame retardant thermoplastic polyester resins, they induce deterioration of said resins, as they are being molded, giving rise to lowered mechanical strengths and dripping of molten resin at the time of combustion. That antimonates are effective flame retarders is well known by U.S. Pat. Nos. 3,892,667 and 3,897,389, for example. However, use of antimonates having high hygroscopicity, for example, sodium antimonate, in flame retardant thermoplastic polyester resin compositions will fail to provide flame retardant resin compositions having both high mechanical strengths and excellent thermal stability. Against this background, there is a strong call for flame retardant thermoplastic polyester resin compositions having relatively high mechanical strengths, involving no practical problem in regard to hygroscopicity and also being excellent in thermal stability.